Level measuring instruments using various technology are known. Certain applications necessitate the use of a level measuring instrument which does not come into contact with the material the level of which is being measured. One such device is an ultrasonic measuring system in which an ultrasonic transmitter is vibrated to generate an acoustic signal directed at the material. A return signal is received by an ultrasonic receiver. In one known form, an acoustic transducer is used in which common components are used for both the transmitter and receiver operating in a pulse echo mode. A crystal is pulsed to generate an acoustic sound wave. The crystal is then de-energized and the acoustic sound wave echoes off the material and is received by the transducer, with the time difference between transmission and return of the echo representing distance, and thus level.
Numerous problems exist with respect to the design of such ultrasonic transducers. For example, an optimum impedance matching material must be used to efficiently transmit sound waves at ultrasonic frequencies from a piezoelectric crystal into air.
Martner, U.S. Pat. No. 3,804,329, discloses an ultrasonic generator for use as an atomizer of liquids. A large diameter disk is clamped to a small annular crystal. This disk vibrates in what is known as the flexural mode. When all parts are vibrating in phase the disk vibrates in a mode shape having node and anti-node areas located in concentric rings radiating from the center of the disk. Particularly, the disk is oscillated so that different circular areas of the plate, referred to as nodes, do not vibrate. The nodes separate adjacent positive anti-nodes and negative anti-nodes, which oscillate oppositely. However, the plate is of a high acoustic impedance material, while the environment in which it is typically used is of low acoustic impedance which results in poor transmission of energy from the plate to the medium. Moreover, after traveling a short distance, the wave fronts from the positive and negative anti-nodes cancel each other since they are 180.degree. out of phase.
Various solutions have been proposed for solving the problems evident with the Martner ultrasonic generator when used as a level measuring device. Panton, U.S. Pat. No. 4,333,028, discloses the use of a flexural mode transducer using impedance matching and phase shifting rings to increase sensitivity. The tings are of different thicknesses. The transducer is more expensive to construct and may have less accurate directability. Moreover, when exposed in a hostile environment the non-uniform matching surface can pose its own problems. For example, in a dusty environment the dust will not cover the transducer uniformly because it can collect in the grooves formed by the rings. A nonuniform layer of dust will distort the beam more drastically than is desired. The different thicknesses of the rings at the positive and negative anti-node are used to shift the phase of the signal from the negative anti-node areas by 180 degrees so that it will add to that from the positive anti-node. However, depending on the properties of the acoustic foam material used, which can change with humidity, temperature, etc., the efficiency of the transducer may decrease by making the phase shift different from 180 degrees.
Steinbrunner et al., U.S. Pat. No. 4,768,615, discloses an acoustic transducer using a perforated plate over the vibrating disk to provide a barrier to the sound waves in the negative anti-nodes. As a result, all wave fronts transmitted into the air are in phase to eliminate cancellation. However, the lack of an impedance matching material results in less than optimum sensitivity of the resulting transducer system.
The disclosed invention is directed to overcoming one or more of the problems discussed above in a novel and simple manner.